1. Field of the Invention
The present invention generally relates to a non-volatile memory, and more particularly to a method of twice programming a multi-bit per cell non-volatile memory with a sequence.
2. Description of Related Art
Flash memory is a non-volatile solid state memory device that can be electrically erased and reprogrammed. Conventional flash memory stores a single bit of information in each memory cell such that each memory cell can be programmed to assume two possible states. The conventional flash memory is thus commonly referred to as single-bit per cell flash memory. Modern flash memory is capable of storing two or more bits of information in each memory cell such that each memory cell can be programmed to assume more than two possible states. The modern flash memory is thus commonly referred to as multi-bit per cell flash memory.
In the multi-bit per cell flash memory, data of different state are programmed to the flash memory by storing different amount of charge in the floating gate of the flash memory. As the charge in the floating gate specifically determines the corresponding threshold voltage, the data can then be read from the multi-bit cell per flash memory according to their different threshold voltage. Due to variations among the memory cells during the manufacture, operation or according to other factors, the threshold voltage of each state is not a constant value but a range.
However, conventional multi-bit per cell flash memory, particularly the three-bit per cell or even more-bit per cell flash memory, suffers from floating-gate coupling effect and retention effect. As a result, the conventional multi-bit per cell flash memory could probably result in read errors due to narrow read margin, and a need has thus arisen to propose some novel schemes to improve floating-gate coupling effect and retention effect.